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Phase separation in duplex stainlesssteel: characterization and mitigation

Time: Fri 2022-09-30 09.00

Location: D37, Lindstedtsvägen 5, Stockholm

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Jianling Liu , Egenskaper

Opponent: Professor Aniruddha Biswas, Bhabha Atomic Research Centre, Indien

Supervisor: Professor Peter Hedström, Strukturer, Egenskaper; Professor Joakim Odqvist, Strukturer; Dr. Wangzhong Mu, Strukturer

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Duplex stainless steel (DSS) is a category of widely used stainlesssteel, which are strategically important in a variety of applications such asin the food industry, chemical engineering plants and nuclear power plants,due to their attractive combination of mechanical properties and corrosionresistance. However, these steel grades are sensitive to the so-called ‘475°Cembrittlement’, because of a phase separation (PS) phenomenon in theferrite phase, which decomposes into Fe-rich ferrite (α) and Cr-rich ferrite(α'), when exposed to temperatures within the miscibility gap. The PS isaccompanied by an increase of micro-hardness of the bcc phase and a severeloss of toughness, leading to the deterioration of the mechanical properties.Therefore, the upper service temperature of DSSs in industrial applicationshas been limited to around 250° C.In the present work, the PS in DSSs was mainly investigated by smallangleneutron scattering (SANS). A quantitative analysis method based onthe SANS data was proposed to evaluate both the wavelength and amplitudeof PS in Fe-Cr based alloys, e.g. DSSs. The wavelength and amplitudequantified by this methodology showed a good agreement with the resultsby atom probe tomography (APT). Then, SANS measurements and thismethod were applied for a comprehensive investigation on the applicationof DSSs and their weldments under industrially relevant conditions, i.e.low/intermediate temperature and prolonged aging time, in order to pursuethe structure-property relation. The current study shows that the measuredCr amplitude is connected to the change of micro-hardness and impacttoughness. Moreover, embrittlement is a function of both the isothermalaging temperature and time.In order to find effective ways to mitigate the PS, this work has alsoattempted to investigate the governing thermodynamics and the kinetics ofPS. First, the effects of alloying elements (e.g. Ni, Al, Co) on the Fe-Crsystem were critically reviewed since such understanding could pave theway for the design of the next generation of DSSs that are less susceptibleto embrittlement; Second, the process route can also influence the PS andit was therefore investigated. It was found that a faster cooling rate aftersolution treatment leads to the lower rate of PS. Moreover, applying anexternal magnetic field may also affect the kinetics of PS: an in-situ SANSstudy showed that a 1.5 T applied magnetic field can significantly delay thePS of DSSs in the early stages.This thesis can be summarized in two parts: i) demonstration of SANSfor characterizing and quantifying PS in DSSs under industrially relevantconditions; ii) discussion of the possibility to mitigate PS in DSSs to becomeless susceptible to low/intermediate temperature embrittlement.